The present invention relates to a method for providing a combustible mixture of a liquid fuel and combustion air, as well as a prevaporizing and premixing burner for liquid fuels, having one or several fuel heaters for heating the liquid fuel prior to combustion.
In the field of household and small consumers (HuK) it is known to burn fuel oil EL for heating purposes or for purposes of thermal process technology in a pressure atomizer burner. The liquid fuel oil EL is converted under high pressure (500 to 2000 kPA) into a fog of droplets by means of an atomizer nozzle and is simultaneously mixed with the supplied combustion air. A method exists in addition, wherein fuel oil EL is atomized by means of compressed air. Further than that there are vaporization burner devices, wherein the liquid fuel is vaporized on the surface of a heated body which is surrounded by combustion air.
The following problems are connected with the present-day burners: in conventional oil burners, the liquid fuel oil EL is converted into a fog of droplets by means of an atomizer nozzle and simultaneously mixed with the supplied combustion air. The processes, such as atomizing, mixing, vaporizing and gasification of the fuel, as well as the combustion of the gasified fuel, occur unregulated side-by-side and in an interacting manner. The individual oil droplets are surrounded by a flame envelope. The high temperatures in the vicinity of the drops, together with the simultaneously occurring lack of air, trigger cracking processes, by which soot is formed.
Present-day blue flame burners avoid the generation of soot in that they vaporize the fuel at the flame root prior to combustion. Here hot flue gases returned from the flame zone here vaporize the oil spray emerging from a swirl nozzle. The water content of the returned flue gases prevents the formation of long-chain hydrocarbons, which can only be burned along with the generation of soot. The method of recirculating the exhaust gases lowers the nitrous oxide emissions in addition to the soot emissions. In order to convey a sufficiently large amount of hot flue gas into the flame root, a correspondingly large induction effect of the fuel/air jet within the mixture preparation is required. The induced mass flow is affected for one by the velocity of the emerging mixture flow and also by the cross section of the open jet. Both parameters can only be varied within certain limits. A high outlet velocity leads to loud flow noises, an increased blower output and large burner dimensions. An increase in the outlet cross section, together with a reduction in velocity, leads to ignition conditions already being created in the vaporization area, so that the intended fuel vaporization, which is uncoupled from the combustion reaction, does not occur. Moreover, the pulse exchange between the fuel and the combustion air is reduced, by which the mixture is also negatively affected. In addition, a high outlet velocity at the twist generator prevents a flame formation in the near range of the mixing device and thereby leads to a reduced thermal stress of these components. It follows from this that in connection with present-day mixture preparation methods for oil burners a reduction of the noxious matter emissions is always connected with an increase in the velocity of the combustion air, and therefore leads to increases in noise emissions and the required blower output.
In a burner system having a firing equipment output of 15 kW, a reduction of the fuel oil flow rate is not possible with conventional oil pressure atomizer nozzles. For reducing the throughput, the nozzle cross section cannot be further reduced for reasons of dependability. The pump pressure can also not be arbitrarily reduced, because the atomizing quality is clearly reduced.
Conventional oil burners are heterogeneous systems, i.e. the dispersed phase fuel oil EL and the dispersion medium air exist as discrete phases side-by-side and are separated by a phase boundary. The roughly dispersed fuel distribution caused by atomizing does not make it possible to mix the fuel without prior vaporization in front of the flame, because the individual fuel droplets settle under the effects of gravity and are deposited on the mixing chamber walls. For this reason a premixing surface burner device, such as can be used in the field of gas combustion, is not possible.
Modem gas burner devices show that a reduction of the nitrous oxide emissions is most effectively achieved by means of a premixing burner system.
A gasification device for fuel oil and kerosene is known from German patent DE-C2-24 56 526, and an oil heating device from German published application DE-OS 14 01 756, wherein the fuel is heated prior to atomizing. Although heating the fuel leads to improved and finer atomizing, problems occur because of cracking product depositions, such as clogging of the lines, etc.
The above mentioned problems are solved by the present invention by the provision of a method wherein: the liquid fuel is put under pressure in a heating phase with the fuel valve closed; the fuel under pressure and in liquid form is heated; following the heating phase the fuel valve is opened and the heated liquid fuel under pressure is atomized and vaporized through a nozzle; the vaporized fuel is mixed with combustion air, where at least a part of the vaporized fuel is condensed, so that a colloid-dispersed and/or a molecular-dispersed fuel-air mixture is created; the fuel valve is closed to terminate combustion; and, with the fuel valve closed, the heated and liquid fuel is cooled;
The advantages which can be achieved by means of the present invention consist in particular in that, with the method on which the present invention is based, a colloid-dispersed or molecular-dispersed fuel distribution occurs, depending on the degree of air preheating. A mixed form of both distribution types is also conceivable. Because of the stability of the colloid-dispersed fuel distribution, it is possible to mix the reactants ahead of the flame without the fuel droplets being deposited on the mixing chamber walls. Here, the mixing of the reactants is possible completely spatially decoupled from the combustion reaction, and not, as with conventional emission-reducing oil burners (so-called blue flame burners), only inside a very small gasification zone ahead of the flame, which is in direct convective heat exchange with the flame via the flue gas recirculation. Because the mixture of fuel and combustion air is now no longer limited to the gasification zone ahead of the flame, the premix burner devices known from gas burner technology, which make possible a very intensive mixing of the reactants, can now also be employed for liquid fuels. The known advantages of this burner technology are therefore also now usable for liquid fuels. Among these are:
(a) low emissions (soot, nitrous oxide, carbon monoxide) when using a surface combustion system;
(b) low noise emissions;
(c) small blower output required;
(d) a combustion air blower system can possibly completely omitted (atmospheric mixture formation);
compact heat generator construction because of the direct coupling of the heat exchanger on the heating cycle side to the reaction zone, which can be spatially exactly determined.
The core of the prevaporizing, premixing combustion technique is constituted by the heating of the liquid fuel oil under pressure. Vaporization of the fuel oil only takes place at the nozzle outlet in contrast to conventional vaporization burner devices, wherein the oil impinges with almost no pressure on a hot surface, which results in the deposition of low-volatile fuel oil components. Maintaining the above mentioned pressure conditions in the operational phases, in which heated fuel oil is in the hydraulic system of the burner, prevents these deposits. Both during the heating phase and during the cooling phase, the oil lines from the pump to the heated fuel valve are pressure sealed, or the pressure in the system is maintained by means of an oil pump or of a compensation vessel (for example metal bellows).
A fuel valve with xe2x80x9catomizing characteristicsxe2x80x9d is used in the system in accordance with the present invention, which unblocks the nozzle opening starting at a defined pressure. The oil vaporization triggered by the pressure reduction at the valve outlet causes an extreme increase in volume, and therefore a considerable reduction of throughput in comparison with the operation of the fuel valve with fuel oil which was not preheated. When using a swirl nozzle, a reduction of the throughput is caused by the reduction in viscosity connected with preheating the fuel. The air core within the nozzle opening increases with increasing fuel temperature and the fuel throughput decreases. The extreme preheating makes it possible to design the nozzle opening considerably larger, in particular with small throughputs, than would be possible with conventional pressure atomizer systems. Because of employing the twist principle in a return flow nozzle with an integrated needle valve, the required output of the firing equipment can be further reduced.
In a further development of the present invention it is provided that the heating device is constituted by at least one electric heating rod, heating element or heating cartridge. The heating device is designed in such a way that at maximum throughput the fuel is heated to the desired temperature. It is advantageous to provide the fuel valve additionally with a temperature sensor, for example a thermal element or the like, so that its temperature can be detected for regulating the heating output of the heating device.
A particularly simple exemplary embodiment provides that the heating device is placed into the fuel valve. In this case the individual heating cartridges or the like can be inserted into bores, for example. However, it is also conceivable that the heating device can be installed on the fuel valve, for example flanged to it, so that there is a direct contact between the heating device and the fuel valve.
With one exemplary embodiment, the fuel valve is embodied as a simplex nozzle with a closing piston. In this case the closing piston can be located outside or inside of the fuel valve.
A further development provides that the fuel valve has a return flow opening and can be combined with a return flow line. A return flow system is created in this way and the fuel valve is used as the return flow nozzle.
When the hydraulic system is laid out as a return flow system, direct electric heating of the valve, or respectively of the valve body, is not necessary. It is sufficient to heat the fuel by means of an electrically heated fuel heater which is remote from the fuel valve and is arranged upstream of the fuel valve, viewed in the flow direction. Transferring the fuel by pumping at a small pressure difference between the forward flow and return flow pressure prior to opening the valve causes heating of the fuel volume inside the valve. In this way the emergence of insufficiently heated fuel immediately following the opening of the fuel valve is prevented.
The return flow nozzle can a for example a have an integrated needle valve, which pressure-seals the nozzle opening during the heating and cooling phase. The movement of the valve tappet is made possible by means of the pressure difference between the forward and return flow pressure. Transferring the fuel oil by pumping at a small pressure difference between the forward flow and return flow pressure prior to opening the valve prevents the emergence of insufficiently heated fuel oil. For cooling the hot returned oil mass flow it is possible to additionally provide an oil cooler, which heats the combustion air, upstream of the pump inlet. Depending on the degree of air preheating, the proportion of the gaseous fuel in the fuel/air mixture increases. The pulse exchange between the combustion air and the fuel, which affects the quality of the mixture, also increases with increasing air temperature.
A further development provides that an adjustable flow resistor for pressure regulation, as well as an adjustable check valve, are provided in the return flow line.
With a return flow line which is merely used as a leakage line, no special measures for cooling the very small oil mass flow are necessary. For example, with a coaxial combination of the oil feed line and the oil return line, the cooling action of the fed-in oil mass flow is sufficient. Finally, embodiments are known, with which a return flow line is not required.
A burner with a fuel valve terminating into free space immediately after the valve tappet has the advantage that, depending on the degree of air preheating, a colloid-dispersed or molecular-dispersed dispersed fuel distribution occurs. Because of the stability of the colloid-disperse or respectively the molecular-dispersed distributed fuel it is possible to mix the reactants already ahead of the flame in an area of large volume without the fuel droplets being deposited on the mixing chamber walls. Therefore, mixing of the reactants is possible completely spatially decoupled from the combustion reaction, and not, as with conventional emission-reducing oil burners (so-called blue flame burners), only inside a very small gasification zone ahead of the flame, which is in direct convective heat exchange with the flame via the flue gas recirculation. The low temperature of the quasi-homogeneous mixture of the burner in accordance with the present invention permits intensive mixing in a mixing zone of large volume without the danger of spontaneous ignition. Now the mixing of fuel and combustion air is no longer limited to the gasification zone ahead of the flame. By employing a return flow nozzle in connection with extreme preheating of the fuel under pressure in particular, a small required firing equipment output can be achieved with operational dependability. Moreover, a large advantage is achieved in that deposits of cracking products are prevented, since the fuel vaporization takes place in the free atmosphere and not, as in film vaporization burners, at a hot surface in the presence of oxygen.
The heating zone is located in the direct vicinity of the reaction body, but is spaced apart from it. With another exemplary embodiment, the heating zone is connected directly with the reaction body. By means of this embodiment the fuel is heated during its passage past the heating zone by the reaction body which, as a rule, is red hot during operation. Therefore separate heating devices are not required during operation. In this case heating can be accomplished by means of radiated energy, by means of convection or by direct contact by means of heat conduction.
With a particularly preferred exemplary embodiment the heating zone is designed as a ring conduit. In this way a comparatively large surface for the inflowing fuel is created, so that it can be rapidly heated, for example by radiation. With a sleeve-like reaction body enclosing the ring conduit in particular, a very large surface for heating is available.
With another embodiment it is provided that the heating zone is designed as a spiral tube. The fuel to be heated is conducted through this spiral tube, wherein the reaction body directly radiates against the spiral tube.
In the preheating phase prior to the start of the burner, the fuel is heated in that an electric heating cartridge is provided, which is connected with the heating zone. In particular, the heating zone rests directly against the heating cartridge, so that the heat from the heating cartridge is transferred by heat conduction to the heating zone and from there to the fuel. In this case the heating cartridge can be designed as a heating rod or a heating spiral.
In a preferred embodiment, sections of the heating cartridge are in connection with the area through which the fuel-air mixture is conducted, wherein a flashback arrester is provided in the direction toward the mixture preparation. In this exemplary embodiment the heating cartridge is additionally used as an ignition device, wherein the fuel-air mixture is ignited on the casing of the heating element, which as a rule is red hot. Separate ignition devices are therefore superfluous.
Further advantages, characteristics and details ensue from the claims as well as from the following description, in which several preferred exemplary embodiments and variations are described in detail, making reference to the drawings.